1. Field of the Invention
This invention relates to a motor-driven camera of the prewind type in which all the frame portions of film are preliminarily wound onto a take-up spool and then the film is rewound for every photographing shot.
2. Description of the Related Art
The camera of the so-called prewind type in which all the frame portions of film are first wound up and, after that, a photographing operation is performed by rewinding the wound-up film has been known and dis closed, for example, in U.S. Pat. No. 4,460,025. The camera of this type must be arranged to accurately count the number of frames during the prewinding action. If not, the photographing operation either would be performed including a film part which has been exposed to light when the camera is loaded with the film or would be prematurely terminated leaving some film part which still remains unexposed and photographable. This might lead to a serious drawback of the photographing operation. Therefore, during the prewinding action, the number of frames must be accurately detected by some suitable electrical means.
The film amount is preferably detected by phase detecting means for accurately controlling a frame indexing action on every one of the frame portions of the film, because: In photographing, frame indexing to be done by an automatic film transporting process is controlled by this detecting means. Therefore, the utilization of the same detecting means obviates the necessity of use of any special additional means for the above-stated purpose. However, the prewinding action is preferably accomplished within a short period of time by rotating a motor at a high speed. Hence, at the end of the prewinding action, the film is in a stretched state. After a current supply to the motor is cut off (or even if the current supply to the motor is allowed to continue), a reaction to the stretching force causes the film to be rewound to some extent toward a film cartridge. The rewound extent tends to give wrong information to the phase detecting means.
In other words, if the phase to be detected is located at the boundary of a phase detecting part at the end of the prewinding action, the phase would come out of the phase detecting part the instant the current supply to the motor is cut off while the counted value of an electric counter has already been incremented by one. Under such a condition, it would be impossible to have the counted value of the electric counter decremented by one if either a power supply to the phase detecting means is cut off or the applicable circuit is no longer in a state of receiving the detection signal from the phase detecting means (for example, when a microcomputer is engaged in a different work).
In such a case, therefore, if the photographing operation is continuously performed to the last, the counted value of the counter which is larger than the actual film length would cause the photographing operation to be performed including a film part which has already been exposed to light by the film loading on the camera.
Further, the camera of the prewind type first winds up the whole film and, after that, performs each photographing by rewinding one frame portion of the film. In this instance, if photographing is performed immediately after completion of the prewinding action, the photographed frame would be located too close to a cartridge part. Then, in cutting the film off from the film cartridge for a film developing process, the cut part might be located inside of the exposed (photographed) frame part. To avoid this, the film must be rewound to a predetermined extent upon completion of the prewinding action. If the predetermined extent is too large or too small, there arises the following problem:
If the predetermined extent is too large, it might become impossible to obtain the number of photographing frames specified on the film. If the predetermined extent is too small, some of photographed frames might be cut. Further, although the camera is provided with some phase detecting means for controlling a frame indexing action on every frame portion of the film, it is impossible to locate the phase of the phase detecting means at the end of the prewinding action.